A known clutch arrangement includes friction plates enclosed in a clutch housing. The friction plates are compressed mechanically or pneumatically by a circular piston pressing on a pressure plate, which comes into contact with the friction plates and pushes them together. An output shaft having the load to be driven attached to it, e.g. by a coupling, has alternate friction plates in mechanical cooperation with it. The other alternate friction plates (i.e. those not in cooperation with the output shaft) are in mechanical cooperation with a driving sleeve which rotates as part of the driving shaft. The clutch works by friction acting between the friction plates as they are pushed together.
GB 2216203 discloses an example of the above type of engagement device. It describes an internally splined driving sleeve movable under the action of a pneumatic ram to engage an output dog drivably connected to an output shaft—this engagement effectively makes the driving sleeve and output shaft a single mechanical member, thereby avoiding the dependence on operating air pressure. The ram has an actuating rod with a fork element attached to it, the fingers of the fork element engaging an annular groove in the outer surface of the driving sleeve. Thus, when compressed air acts on an end of the pneumatic ram, the actuating rod slides axially, moving the driving sleeve with it. The driving sleeve has a pressure plate located inside it and releasably engaged to it by means of steel balls resiliently urged into depressions formed in the inner surface of the sleeve. There are a set of friction plates, alternate ones of which are engaged with the internal splines of the driving sleeve, the remainder being engaged with the output shaft. When the sleeve is initially moved towards engagement with the output dog, the pressure plate moves axially with it and loads the friction plates against one another to begin turning the output shaft. A large torque is required to start the rotation because of the inertia of the load attached to the output shaft. This torque manifests itself as friction between the friction plates and the internal splines of the driving sleeve. This friction is enough to prevent further sliding motion of the driving sleeve until the rotational speeds of the driving sleeve and output shaft are more or less equal. The torque required to turn the output shaft is then less, so the friction acting on the splines of the driving sleeve is reduced and sliding recommences.
In WO 2004/109137, the present inventor proposed a improved arrangement of the type described in GB 2216203 wherein the functions of (i) engaging a driving member with a driven member, and (ii) activating a friction drive e.g. by pushing friction plates together were separated by incorporating a valve arrangement in the pneumatic ram, which allowed a force to act through the ram without necessarily moving the driving member. Thus, the clutch was used only to synchronise the driving and driven members, which meant that it was less likely to burn out through overloading. The valve arrangement in the piston also allowed an air controlled friction drive to be deactivated just before the moment of positive engagement of the clutch, in order for engagement to proceed smoothly.
The driving member and driven member typically have toothed projections which interlock to provide the positive engagement. On rare occasions, the driving member and driven member are synchronised so that the toothed projections become aligned and abut one another at the point of engagement, i.e. instead of intermeshing, the projection are pressed together point-to-point. One disadvantage of this is that positive engagement is not properly achieved because there is no physical interlock between the driving member and the driven member. There is therefore a risk of the driving member and the driven member slipping relative to one another, e.g. if the size of the driven load reduces for any reason. Such slipping can jar the apparatus, and may cause damage e.g. to the interlocking features. Furthermore, when engagement is missed the pneumatic ram is prevented from moving along its full axial extent but continues to be urged towards that position by the air pressure. This puts a load on the connection between the pneumatic ram and the element (e.g. fork element) that connects the ram to the member (driven or driving) which it moves. This connection may be weakened or otherwise damaged (e.g. bent out of alignment) by this force.